In the electrophotographic image forming method widely used in these days, a charged toner is brought in contact with or provided to oppose via a narrow interval an electrostatic latent image which is formed on an electrostatic latent image carrier (normally an electrophotographic photoreceptor) to visualize the electrostatic latent image by the toner, which is referred to as a developing process, thus a toner image is formed. Then, the toner image formed on the electrostatic latent image carrier is transferred to a regular paper sheet, followed by fixing, to obtain a final image.
As the method for forming the toner image, there are known (i) a two-component developing method in which a two-component developer formed from a carrier and a toner are used to charge the toner and perform development; and (ii) a one-component developing method in which only toner is used and the toner is charged by friction with the developing roller which conveys the developer or with the developer controlling member and the like to perform development. In the one-component developing method, since no carrier is used, the developing apparatus can be simplified, and thus it has been very widely used in recent years. The one-component development method has been receiving much attention in the recent trend of colorization because a non-magnetic one-component development method in which the toner contains no magnetic material can also be colorized.
In this method, unlike the two-component development method, carrier is not used and only toner is subjected to friction using the charging member, or alternatively the toner is charged by pressing on the developing roller surface, and thus this has great merit in that the structure of the developing device is not complex and can be made compact. As a result, it can be easily applied to color image forming apparatus which requires four or more developing mechanisms. In recent years in particular, progress has been made in making the devices lighter and more compact, and in printers, developing systems which use non-magnetic single-component developers have become a main stream.
In the non-magnetic one-component development system that have been used so far, a developing roller having an elastic layer made of silicon rubber at the outer periphery of the conductive shaft have been used. The charging of the toner is carried out by forming a thin layer of the toner on a developing roller using a charging member such as a metal plate or a roller and by causing friction with the layer. Accordingly, a developing device with an extremely simple mechanical structure is obtained.
The developing roller has an elastic layer made of, for example, silicon rubber on the outer periphery of a conductive shaft made from a metal or a conductive resin, and a surface layer may be formed on the elastic layer in order to impart charging property to the toner or to impart conveyance properties to the toner. As the surface layer, it has also been known that a fluorinated rubber can be used to prevent the toner from adhering to or fusing with the surface layer. In order to form the fluorinated rubber layer on the elastic layer, adhesive properties must be improved and a method of forming the elastic layer surface with an intermediate layer of a silane coupling agent and further forming a coating layer which has fluorinated rubber as its main component thereon is known (for example, see Patent Document 1).
Due to the more compact size and shorter printout times of recent devices, the cycle of toner conveyance, charging, development and toner replacement which are functions of the developing roller has become faster and the load of the developing roller has become increased.
In the developing roller which has a structure in which a coating layer is formed directly on the shaft, a stronger adhesion of the coating layer to the shaft is needed, since when a force is applied from the outside to the coating layer, the shock cannot be absorbed unlike in the case of the developing roller in which a coating layer is provided via an elastic layer.
Providing an adhesive layer between the shaft and the coating layer in order to improve the adhesive properties between the shaft and the coating layer has been examined (for example, see Patent Document 2).
In addition, using stainless steel to form the shaft and forming a coating layer directly on the shaft has been examined (for example, see Patent Document 3).
However, in the developing roller which has a structure in which a coating layer is formed directly on the shaft as mentioned above, the adhesion of the coating layer to the shaft has not been strong enough, and it has not been fully easy, when a large number of images are printed, to continuously obtain high quality toner images due to peeling of the coating layer from the shaft.
Patent Document 1Unexamined Japanese PatentApplication Publication (hereafterreferred to as JP-A) No.8-190263Patent Document 2JP-A No. 7-56434Patent Document 3JP-A No. 2002-14535